The general rule of thumb is to always include a cover letter with a resume. In fact, the Quintessential Careers career advice site says you should never send a resume that isn't accompanied by a cover letter. However, there are rare exceptions to this rule, depending on an employer's specific instructions and whether technology allows for cover letters to be submitted.
A primary function of your cover letter is to introduce yourself to the hiring manager and state the purpose of your submission. The cover letter should be specific to a posted job opening and give you a chance to state your interest in the job and draw attention to your qualifications. Closely related to a cover letter is a letter of interest, which you send with a resume to express interest in unadvertised job opportunities.
Highlight Compelling Qualities
A cover letter is a great opportunity to highlight compelling qualities or to tell a story that isn't clear in a resume. Someone making a career change, for instance, can use a letter to explain recent education or training she has completed in preparation of the application. You also have a chance to highlight a handful of strengths that are important to the hiring manager, and set the stage for personal branding that you carry in to the interview. During an initial human resources application review, it is also possible that some of your experiences or qualifications are clarified in your letter more than in your resume.
Employer requirements or expectations of a cover letter vary. When an employer directly calls for a cover letter, your submission is incomplete if you don't include one. In some cases, a job listing might not mention that a cover letter is required, but submissions without letters aren't reviewed. Even if the employer isn't going to discard a resume without a cover letter, not sending one means missing an opportunity to catch the hiring manager's eye before he scans your resume. Some companies no longer require a letter with an application and resume, according to a September 2013 article in Science magazine. In cases where a company specifically mentions no cover letter, don't send one.
Technology Roadblocks and Exceptions
Exceptions to the general rule of including a cover letter are rare. Again, if a hiring manager specifically advises you not to include one, then don't. The more typical reason someone wouldn't include a cover letter is because he can't. Many online application tools don't allow for the submission of a cover letter, according to a November 2013 "USA Today" article. Also, when you use networking to pursue a job opportunity, you might submit a cover letter because the interview is arranged with a more personal, informal approach.